1. Field of the Invention
The present invention relates to a radiographic intensifying screen and a radiation image producing method utilizing a screen-film system.
2. Description of the Prior Art
In a various kinds of radiography such as medical radiography for diagnosis and industrial radiography for nondestructive inspection, a radiographic intensifying screen is generally employed in close contact with one or both surfaces of a radiographic film such as an X-ray film.
The radiographic film comprises a support and an emulsion layer provided on one or both surfaces thereof which comprises a binder and a silver halide particles dispersed therein. The radiographic intensifying screen comprises a support and a phosphor layer provided on one surface of th support. The phosphor layer comprises a binder and phosphor particles dispersed therein. When excited with a radiation such as X-rays having passed through an object, the phosphor particles emit light of high luminance in proportion to the dose of the radiation. Accordingly, the radiographic film placed in close contact with the phosphor layer of the intensifying screen can be exposed sufficiently to radiation to form a radiation image of the object, even if the radiation is applied to the object at a relatively small dose.
In a conventional radiography, a screen-film system comprising a combination of a radiographic film and a radiographic intensifying screen is employed. For example, there are known a screen-film system comprising a radiographic film having an emulsion layer on one side (single-sided film) and a radiographic intensifying screen placed on the emulsion layer-side of the film (i.e., single-sided system or single-sided screen-film system), and a screen-film system comprising a radiographic film having two emulsion layers on both sides (double-sided film) and two radiographic intensifying screens (a front screen located on the radiation impinging side and a back screen located on the opposite side of the radiation impinging side) placed on both sides of the film (i.e., double-sided system or double-sided screen-film system).
Generally, the quality of an image (i.e., sharpness, graininess, etc.) obtained by the screen-film system is greatly influenced by the characteristics of a radiographic intensifying screen used in the system, and it is highly desired for the screen to provide an image of high quality.
For enhancing the sharpness of an image, there has been proposed a radiographic intensifying screen comprising phosphor particles having larger diameters arranged on the screen surface side of the phosphor layer (i.e., the side from which the emitted light is detected) and phosphor particles having smaller diameters arranged on the support side of the phosphor layer, as described, for example, in Japanese Patent Publication No. 55(1980)-33560 and Japanese Patent Provisional Publication No. 58(1983)-71500. Otherwise, Japanese Patent Provisional Publication No. 58(1983)-160952 describes as the above-mentioned single-sided system a screen-film system using a radiographic intensifying screen which comprises larger particles of a rare earth element phosphor arranged on the side near the film and smaller particles thereof arranged on the farther side from the film to provide an image of improved sharpness without decreasing the radiographic speed. In a single-sided system employing such radiographic intensifying screen, the radiographic speed can be improved because the particle diameters of the phosphor are relatively large on the side near the film. In other words, the sharpness of an image provided by the system can be enhanced when the radiographic speed of the screen used in the system is the same as a conventional screen having a uniform particle diameter distribution in the direction of thickness of the phosphor layer. Further, the phosphor particles of small diameters arranged on the support side have the same function as that of a reflecting layer, and accordingly the emitted light can be readily detected from the surface of the screen by shortening the passage of reflected or scattered light of the emission, whereby the sharpness of an image (in the low frequency region) can be enhanced.
Also in the case of a double-sided sytem comprising radiographic intensifying screens arranged on both sides of the radiographic film in which the phosphor layer of each screens has the above-mentioned specific particle diameter distribution, the same tendencies as described in the single-sided sytem are observed.
Generally, the radiographic speed of a radiographic intensifying screen becomes higher as the particle diameter of the phosphor is larger, while the sharpness and graininess of an image provided by the screen are more improved as particle diameter thereof is smaller.